Effects of soil water heterogeneity on water relations of two shrubs, Ambrosia dumosa and Encelia farinosa (Asteraceae)

Many desert shrub species split morphologically into independent hydraulic units (IHUs) as they mature. IHUs are hypothesized to be an adaptation to living in dry and heterogeneous soil-water environments, because segregation of a plant's hydraulic system may prevent spread of embolisms and may decrease root water loss reducing hydraulic redistribution. Division into IHUs occurs in many shrubs in the Sonoran and Mojave Deserts, including Ambrosia dumosa. However, some species, such as Encelia farinosa, never physically split and appear to maintain an integrated hydraulic system. Hydraulic architecture and hydraulic redistribution were compared between young Ambrosia and young Encelia shrubs. I hypothesized that heterogeneous water supply would cause significantly more spatial variation in water use within Ambrosia canopies than within canopies of Encelia and that root systems of Encelia shrubs would have higher hydraulic redistribution than root systems of Ambrosia shrubs. Young shrubs of the two species were planted in containers designed to allow heterogeneous and homogeneous watering treatments. Spatial variation of leaf water potentials and stomatal conductances within the canopies of the two species as well as soil water contents were measured in response to soil water heterogeneity. Results show that young Ambrosia shrubs possess IHUs long before they physically split, and that young Encelia shrubs possess integrated hydraulic systems. No significant hydraulic redistribution was detected for Encelia and Ambrosia shrubs. These findings have important implications for the water relations of these species and their survival in environments with high degrees of temporal and spatial heterogeneity in water availability.